System for automatic frequency control

ABSTRACT

A system for bringing the frequency of an oscillator under the control of the frequency of the carrier of a pulse wherein a parameter characteristic of the carrier frequency value is measured throughout the duration of the pulse, the measurement result is stored, the same parameter is measured in respect of the frequency to be controlled, and the control is carried out after the pulse by comparison of the second measured parameter with the first and stored measured parameter.

BACKGROUND OF THE INVENTION

This invention relates to systems for bringing the frequency of anoscillator under the control of the frequency of the carrier of a pulseof very brief duration.

This is a problem arising in the receivers of systems providingelectromagnetic detection of very brief pulses where pulse frequencyvaries between individual pulses, for in such cases the local oscillatorfrequency of the receiver must also vary but under the control of thevariation in the frequency of the pulse, so that the receiverintermediate frequency remains close enough to its rated value to becompatible with the receiver pass band.

The known techniques can provide such a form of control for the durationof the pulse but cease to be satisfactory for pulse duration below 1microsecond.

The control system according to the invention is of use for much shorterpulses, for instance, lasting 0.25 microseconds.

SUMMARY OF THE INVENTION

According to a feature of the invention, the system comprises means formeasuring and storing a parameter characteristic of the carrierfrequency value throughout the duration of the pulse, means formeasuring the same parameter in respect of the frequency of theoscillator to be controlled, comparing means for comparing after thepulse the second measured parameter with the first and stored measuredparameter and control means for controlling the frequency of saidoscillator as a function of the result of said comparison. Consequently,only the recording of the frequency takes place within a time limitdetermined by pulse duration and such recording can of course be madevery rapidly.

The invention also relates to automatic frequency control systems usingthe method.

DESCRIPTION OF THE FIGURES OF THE DRAWINGS

The invention will be better understood from the following descriptionand by reference to the drawings wherein:

FIG. 1 is a block schematic diagram showing the method according to theinvention;

FIG. 2 is a variant of the method shown in FIG. 1;

FIG. 3 shows a first form of a system using the method according to theinvention;

FIG. 4 shows a variant of the system shown in FIG. 3;

FIG. 5 is a first improvement in the control circuit according to theinvention;

FIG. 6 is a preferred variant of FIG. 5;

FIG. 7 is a first example showing how the system according to theinvention is of use in a varying-frequency radar;

FIG. 8 shows a second improvement of the control circuit according tothe invention, of use in an electromagnetic detection receiver;

FIG. 9 shows a variant of FIG. 8;

FIG. 10 shows an example of a microwave frequency discriminator with avariable central frequency;

FIG. 11 shows an amplitude control loop, and

FIG. 12 shows an example of a medium-frequency frequency-to-voltageconverter with a variable central frequency.

DETAILED DESCRIPTION OF ONE EMBODIMENT

Referring to FIG. 1, there can be seen a frequency-controlled oscillatorO₁ and an input A; available thereat is a pulse to whose carrierfrequency the frequency of oscillator O₁ must be controlled. FrequencyF₃ of the pulse is measured by a frequency-measuring device D₂.

The measurement result is stored in a memory M which is zero reset byeach fresh pulse. The zero reset is performed by a device R. Thefrequency of oscillator O₁ is measured in a device D₁ of the same kindand properties as the device D₂. The results of the two measurements arecompared in a comparator circuit CP which supplies the control signalfor control device CT of oscillator O₁.

The foregoing outline is very broad and says nothing about orders ofmagnitude of the frequencies used nor, therefore, of the nature of thevarious devices, its purpose being to show the method according to theinvention, the main feature thereof being that the time taken to controlthe local oscillator is independent of pulse duration since such controloccurs basically after the pulse, the value of the frequency thereofbeing stored.

As a rule, the oscillator frequency F₁ and the pulse frequency F₃ shouldnot be equal but should differ from one another by a constant valuewhich can be introduced into the circuit by frequency changing eithervia the comparator or before the measurement of at least one of thefrequencies. Advantageously, frequency measurements are performed bymeans of voltage-to-frequency converters or frequency discriminators;their voltage/frequency characteristics must be absolutely identical.

To overcome this difficulty and to simplify the circuit while using thesame measuring device D₁, the system shown in FIG. 2 can be used, wherelike elements have the same references as in FIG. 1. A selector SW₁,controlled e. g. by the pulse, connects the input of the device D₁either to input A during the presence of the pulse or to the oscillatorO₁ after the pulse; synchronously with selector SW₁ a second selectorSW₂ connects the output of the device D₁ either to store M, which inthis case is disposed between the device D and the comparator, or to thecomparator input not connected to the store. In this case, if thefrequencies F₁ and F₃ differ from one another by a constant value theremust be a frequency change before the selector SW₁.

FIG. 3 relates precisely to such a case which is, for instance, a systemfor random-frequency electromagnetic detection. Like elements have thesame reference as in the previous drawings. In this case the oscillatorO₁ is the local oscillator of the receiver and the received echos offrequency F₃ (which is almost the Doppler frequency F_(d)) are mixed infrequency changer X₃ with the signal of frequency F₁. The differencefrequency F₃ - F₁ must therefore be kept constant at the intermediatefrequency F_(i) of the receiver amplification and tuning circuits. Thefrequency F₃ varies around a mean frequency F₃.0 and F₃ can beconsidered equal to F₃.0 + Δ F where ΔF, whether positive or negative,is small in relation to F₃.0. The circuit for controlling the frequencyF₁ to the frequency F₃ is framed in chain lines. Two oscillators O₂, O₄working on fixed frequencies F₂, F₄ respectively differing from oneanother by the intermediate frequency F_(i) provide the auxiliaryfrequency-changing oscillations for changing the frequency of thelocal-oscillator signal and the frequency of the pulse to the samefrequency. Accordingly, a mixer X₁ outputs a signal of frequency F₄ - F₃and a mixer X₂ outputs the signal of frequency F₂ - F₁. In this case thefrequency measuring device D₁ is a frequency discriminator centered onthe frequency F_(i). The frequency F₄ is so chosen that F₄ = F₃.0 +F_(i), so that F₄ - F₃ can be put equal to F_(i) + ΔF and F₂ - F₁ can beput equal to F_(i) + ΔF₁ where ΔF₁ is positive or negative according tocircumstances.

During the life of the pulse when the selectors are in their bottomposition, the frequency difference ΔF leads to the discriminator D₁outputting a voltage ΔV proportional to ΔF stored in the store M (acapacitance in the present case). The store output is applied to one ofthe inputs of the comparator CP, for instance, a difference amplifier.When the pulse has ended and the selectors are in their top position,the discriminator supplies a voltage ΔV₁ proportional to ΔF₁.

If the local oscillator frequency is positioned correctly -- i.e., ifF₃ - F₁ = F_(i), then ΔF₁ = ΔF, since F₂ - F₄ = F_(i) and so ΔV₁ = ΔV:No control voltage is applied to the control device CT, represented by avariable-capacitance diode, and the frequency F₁ does not alter.However, if ΔF = ΔF₁, the amplifier CP outputs a voltage proportional toΔV₁ - ΔV₂ which changes the frequency of oscillator O₁ until ΔF₁ = ΔF.

Storage of the frequency-proportional voltage signal output by thefrequency discriminator entails two contradictory requirements. Thecharging time of the capacitance must be at most 1/3 to 1/4 of pulseduration so that the peak pulse value may be recorded, yet thecapacitance discharge time which governs the stability of theintermediate frequency throughout the whole repetition must be from 300to 400 times greater than the recurrence. Cascading stores to chargelarger capacitors considerably increases switching difficulties anddifficulties connected with interfering signals inherent in the circuitarrangement.

To obviate these disadvantages the difference storage circuitarrangement which is shown in FIG. 4 and which virtually obviates theeffect of charge losses on the recorded value, can be used. Thoseelements of FIG. 3 which appear again in FIG. 4 have the same referencesas in FIG. 3, but for the sake of clarity the oscillators O₂ and O₄ andthe mixer X₃ are not shown in FIG. 4. A separation has been made betweenthe two separate items which usually make up a frequency discriminator,a tuning and envelope detection element, comprising two channels d₁, d₂centered on the frequencies F_(i) + Σ and F_(i) - Σ where Σ is equal tothe maximum value of ΔF, such channels having symmetricalcharacteristics in relation to the frequency F_(i), and a subtractorsupplies the difference between the signals delivered by the twochannels d₁, d₂. In this variant, one subtractor S₁ is used for thesignal F₂ - F₁ and a second subtractor S₂ is used for the signal F₃ -F₄, the subtractors usually being difference amplifiers. The outputsignals from the discriminator channels are stored in M₁ and M₂ at theinputs of subtractor S₂, which therefore measures the difference betweenthe stored values, such difference being independent of the drifts ofthe stores provided that the same are properly matched. The selector SW₂is therefore replaced by a double selector SW_(2a), SW_(2b).

The remainder of the description relates to improvements of theinvention, of use in all the variants hereinbefore described.

A frequency discriminator is distinguished by a central frequency and apassband -- i.e., the permissible frequency departures from the centralfrequency. As a rule, the amplitude of the maximum frequency swingsallowable for the discriminator is adapted to the maximum frequencyseparation between two consecutive pulses. However, factors such astemperature or component wear may by their variation cause slow changesin the mean value of the frequency of the pulses, in which event thefrequency of some pulses departs from the permissible discriminatorfrequency range and frequency control is disturbed.

According to the invention, the frequency-changing circuit for the pulsecomprises, in association with the mixer X₁, a controllable-frequencyoscillator O₄ whose control input is connected to a circuit for slowlycontrolling the oscillator frequency to the mean value of the pulsefrequency.

FIG. 5 shows a variant of the system shown in FIG. 3 and comprising a"slow" control circuit. In addition to the elements of FIG. 3 there area discriminator D2 centered on the frequency F_(i), a sampling device E,a storage circuit M₃ and an integrator In. The slow control circuit isconnected between the output of mixer X₁ and the control input ofoscillator O₄. The same is therefore used as local oscillator for afirst frequency change in a receiver, the oscillator O₁ being associatedwith a second frequency change.

Operation is as follows. During the life of the pulse the mixer X₁delivers a signal of frequency F₄ - F₃ = F_(i) + Δ F. The difference Fcauses discriminator D₂ to output a voltage ΔV proportional to ΔF andstored in storage circuit M₃ by way of sampler E which enables the storeto charge up during the pulse. The voltage ΔV is applied to integratingcircuit In which prepares a control voltage applied to the oscillatorO₄. The integrator time constant must be high in relation to the pulserepetition rate so that the control voltage is a d.c. voltage whosevalue equals the mean value of the voltage stored in M₃. The frequencyof oscillator O₄ is so offset that the mean value of the frequency ofthe pulses applied to the inputs of the discriminators D₁ and D₂ isF_(i).

Such a circuit has the following features:

The discriminators D₁, D₂ operate at frequencies whose mean value isalways the frequency F_(i) despite possible frequency drifting. The meanvalue of every voltage stored in M₁ or M₃ is always zero. Leakage lossesin the stores M₁ and M₃ are therefore very slight. This slow controlcircuit is of course of use in the system shown in FIG. 4.

FIG. 6 shows a preferred variant of FIG. 5. The discriminator D₂,sampler E and store M₃ are omitted, since the voltage stored in M₃ isidentical to the voltage stored in M₁, the operation of the selectorsSW₁, SW₂ being identical to the operation of the sampler E during thepulse. Integrator In therefore directly connected to store M₁. In otherrespects the system operates in precisely the same way as in FIG. 5.

FIG. 7 shows how the system according to the invention is of use in avariable-frequency radar comprising e.g. a magnetron K supplying pulsesto an antenna R. The transmitted frequency varies from any one pulse tothe next. A reduced proportion of the pulse power is applied to input Aof the automatic frequency control circuit by way of a coupler C. On thereceiving side of the system a circulator G routes the received signalto the input of the radar receiver, which comprises a first mixer X₅receiving both the received signal and the sinusoidal signal output bythe slow control circuit local oscillator O₄, the first mixer X₅delivering a received signal having an intermediate frequency F_(i1). Asecond mixer X₃ receives the signal F_(i1) and the sinusoidal outputsignal of the automatic frequency control circuit local oscillator O₁and delivers a received signal at a stable intermediate frequencyF_(i2). In this kind of radar variation of the pulse frequency isprovided by mechanical action on the magnetron cavity. Appropriate meanswhich are not shown and which are included in a magnetron K delivers avoltage which is in a approximate relationship to the pulse frequency.For instance, if frequency variation is produced by deformation of themagnetron cavity, the last-mentioned means comprise a potentiometeracross which a predetermined and constant potential difference ismaintained. The potentiometer is rigidly secured to mechanical deformingmeans, and so the voltage derived between the potentiometer slider andany one end of the potentiometer is representative of the deformationproduced and hence of the transmission frequency of the magnetron K. Thelatter voltage is used to pre-lock the frequency of the local oscillator0₄ briefly before the transmission of each pulse. Accordingly, a voltageadder circuit A_(d) is interposed between the local oscillator 0₄ andthe integrator In. The pre-locking voltage supplied by the magnetron Kat a particular instant of time is applied to the adder A_(d). Thelatter voltage can be sampled by means of a sampling circuit and astore, neither of which is shown. The resulting pre-locking is coarse.It is an object of the system according to the invention to compensatefor the residual frequency variation.

The slow control acts on the frequency of oscillator 0₄ and comprises amixer X₁ receiving the transmitted pulses via the coupling C, plus theoutput signal of oscillator 0₄. The selectors SW₁, SW₂ are in position Iduring each pulse. Discriminator D₁ receives the output signal of mixerX₁ and supplies a voltage which is stored in the store M₁, the meanvalue of the latter voltage serving to control the frequency of theoscillator 0₄ by way of the integrator In and adder Ad.

Fast control of the frequency of the oscillator 01 is achieved bycomparison of the voltage stored in M₁ with the output voltage ofdiscriminator D₁ when the switches are in position II, the lattervoltage representing the frequency of the oscillator 01 after frequencychanging by means of the mixer X₂ and of the fixed-frequency oscillator02. These controls operate in the same way as in the systems shown inFIGS. 1 to 6.

A disadvantage of the system hereinfore described is the need for doublefrequency changing in the receiving channel. The first frequency changeintroduces a first intermediate frequency whose mean value is constantbut whose instantaneous value depends upon the residual frequencydifference between the transmitter and the local oscillator due tounsatisfactory frequency pre-locking thereof. The fast frequency controltransposes this residual difference in value and sign to the frequencyof the second frequency change local oscillator. The residual error ofthe second intermediate frequency is therefore reduced to tolerablevalues of the order of a few hundred kilohertz.

Because the first intermediate frequency is relatively high, there ismore particularly a slight impairment of the signal-to-noise ratio.Also, when the system according to the invention is used in monopulseradars difficulties arise as regard amplitude, phase, function andfrequency identity of channels.

To obviate these disadvantages the invention provides an automaticfrequency control system which is used in an electromagnetic detectionreceiver and which involves just a single frequency change in thereceiver channel, the automatic frequency control system carrying outall the steps necessary for proper frequency locking of the localoscillator.

The system shown in FIG. 8 is used, for instance, in avariable-frequency radar for direct microwave-frequency instantaneouscorrection at transmission of the frequency of the receiver localoscillator. During each repetition period the local oscillator is lockedto a frequency differing from the transmitter frequency by the selectedintermediate frequency with the required accuracy. Radar echos aretherefore received in the receiver at a substantially constantintermediate frequency. As in the previous figures, like referencesdenote like elements of the system.

Of the transmitter only the magnetron K has been shown; it is connectedto antenna R by way of circulator G. After the circulator the receivercomprises only a mixer X₃ receiving a reference signal from the circuitwhich is framed in chain lines and which comprises the controlledoscillator 0₁ and the automatic frequency control system according tothe invention. The received output signals mixer X₃ are changed to theintermediate frequency F_(i).

The magnetron K operates at an instantaneous frequency F₃ in a frequencyrange of width 2ΔF derived from a control Y acting in accordance with acycle of some tens to hundred of hertz. During each cycle the magnetrontransmits pulses through the agency of a modulator and a general radarsynchronizing device (not shown). The magnetron operating frequency canbe discovered in two ways. It is found in one way by means of a copyingdevice which forms part of the magnetron itself and which delivers ad.c. signal proportional to the magnetron tuning frequency but with arelatively large uncertainty. The copying device takes the form, forinstance, of a potentiometer coupled to the electromagnetic orelectromechanical means for tuning the magnetron cavity. In FIG. 8 thecopy voltage is available at terminal k₁ of magnetron K. The frequencycan also be found directly at the microwave output k₂ of the magnetron,for instance, by means of a coupler and of a frequency discriminator.

The controlled oscillator 0₁ is of the kind whose frequency iscontrolled by voltage. It operates on a frequency F₁ = F₃ + Fi and itmust be able to cover the frequency range 2ΔF covered by the magnetronplus the relative frequency drifting of the magnetron and oscillator.

To this end, the automatic frequency control system comprises afrequency-to-voltage converter D₁ which receives either the signal ofthe oscillator 0₁ after frequency changing by the mixer X₂ andoscillator 0₂, by way of a microwave selector SW₁, or the signal at thefrequency F₃ by way of a coupler C which samples a small proportion ofthe magnetron output signal, of a variable attenuator R and of theselector SW₁. The converter output is connected, via a video frequencyselector SW₂, either to the positive input of a voltage comparator CP orto the negative input of such comparator and to a storage device M₁symbocally represented by a capacitance. The comparator output isconnected to an amplifier and frequency corrector AC whose output isconnected via adder Ad₁ to the frequency control input of oscillator O₁.

All the circuits just described together form the fast frequency controlloop of the oscillator O₁. Slow control of the mean value of the centralfrequency to voltage converter is provided on the basis of the voltagestored in M₁. The latter voltage is applied to an integrating amplifierIn whose output is connected via an adder Ad₂ to a central frequencycontrol input 22 of the frequency-to-voltage converter D₁. The voltageapplied to the latter input is a means of shifting the converter centralfrequency from the frequency at which the frequency measurement is made.FIGS. 10 and 12 show more clearly how this central frequency shift isproduced.

In addition to the foregoing controls there is a means for presettingthe frequency of the oscillator O₁ and the central frequency of theconverter D₁, the presetting using the copy signal a sampler and blockerEb₁ whose output is connected to the adders Ad₁, Ad₂.

Immediately before the transmission of a pulse the copy voltage isstored by the samples Eb₁ and presets the frequency of the oscillatorand the central frequency of the frequency-to-voltage converter toprevent an excessive frequency jump in the control loop. Moreparticularly, the central frequency of the converter remains close tothe transmitted frequency despite the presetting. The operative range ofthe converter is therefore reduced and its efficiency improved. Thepresetting voltage remains constant throughout the transmission andreception period, whereafter it alters again until the next pulse.

The fast frequency control operates in each period for which the presetremains unchanged. The selectors SW₁, SW₂ are in position I for theduration of emission of a pulse. The converter D₁ delivers to the storeM₁ a voltage representing magnetron frequency.

Upon the completion of this step the selectors change over to positionII and the converter D₁ outputs a voltage representing the frequency ofthe output signal of oscillator O₁. The latter voltage is compared incomparator CP with the voltage previously stored in store M₁ and thedifference is amplified by the device AC, whose output is looped back tothe frequency control input of the local oscillator O₁ so as to reducethe amount of such difference.

The system thus looped adapts the output voltage of the converter to thevalue previously stored at the time of transmission of the pulse.Consequently, except for loop error which depends upon loop gain, localoscillator frequency is locked to the appropriate value for radar echoreception.

The selectors SW₁, SW₂ look like electromechanical devices in thedrawings but are in fact electronic devices, the selector SW₁ operatingat microwave frequencies and the selector SW₂ at video frequencies.These devices are familiar to those skilled in the art.

FIG. 9 shows that it is even possible not to use the selectors. Apartfrom their omission, all the other elements of FIG. 8 are used in FIG. 9and operate in the same way. The microwave frequency selector SW₁ isomitted and the output mixer X₂ is directly connected to the input ofthe converter D₁. The signal transmitted by the magnetron is in thiscase applied to the converter via a coupler C. In the absence oftransmission the output signal of mixer X₂ is transmitted to theconverter. During the transmission of a pulse the latter signal issuppressed by the power supply A1 of oscillator O₁ being interrupted bymeans of a switch SW₃.

The selector SW₂ is omitted at the converter output and the converter isdirectly connected to the positive input of the comparator CP. Asampling and blocking device Eb₂ is disposed between the converteroutput and the negative input of the comparator and is synchronized withswitch SW₃. The store M₁ is omitted, its function being performed by thedevice Eb₂.

During the pulse the voltage representing magnetron frequency is storedin the samples Eb₂. At this time the local oscillator stops operating sothat the converter receives only the magnetron signal. After the pulsethe stored voltage applied to the negative input of the comparator iscompared with the voltage output representing the frequency of theoscillator O₁ applied to the positive input. In other respects operationis exactly as for FIG. 8.

FIG. 10 shows an example of a frequency-to-voltage converter orfrequency discriminator operating at microwave frequencies and having acontrollable central frequency. Like most microwave discriminators it isembodied by a 3db directional input coupler 11, one input 10 of whichreceives the microwave signal and the other input of which is connectedto a matched load 12, a 3db directional output coupler 13 whose outputsare connected to detectors 14, 15, and a comparator 16 which outputs toa terminal 9 and, between the two couplers, two microwave connections ofthe same electrical length. One such connection comprises a resonantcavity 17 and the other comprises an adjustable attenuator 18 in serieswith a phase shifter 19 serving to produce and adjust equality of theelectrical lengths and equality of gain of the two connections at thecentral frequency. The discriminator central frequency is determined bythe resonant frequency of cavity 17, which is modified by means of avariable-reactance element 20 such as a variable-capacitance diodecoupled to the cavity and acted on by the frequency control voltageapplied at terminal 22. The latter voltage varies the diode capacitanceand therefore the tuned frequency of the system embodied by the diodeand the cavity. At the output from the detectors 14 and 15 the signalsare of the same phase and of the same amplitude when the signal at theinput 10 has the same frequency as the cavity resonant frequency. Theoutput signal at 9 is zero. Off the resonant frequency the output signalis substantially proportional to the resonant frequency of the cavity.

For this kind of discriminator the amplitudes of the input signals mustbe exactly the same during the two consecutive operations of storage andcombination. This can be achieved either by means of anamplitude-limiting circuit at the discriminator input or by aninstantaneous amplitude control circuit.

The former case can be considered for operating frequencies below 1000MHZ in the present state of the art. Above this frequency a fastamplitude control loop must be used and a case of this kind will now bedescribed with reference to FIG. 11.

The principle on which an amplitude control loop operates is the same asfor a frequency loop. A fast attenuator 40 is interposed between mixerX₂ and selector SW₁. A level detector 41 is connected to selector SW₁and, by way of a selector SW₄, supplies a signal either to a store 44and the negative input of a comparator 43 (position I) or to thepositive input of the comparator (position II). An amplifier andcorrector 45 closes the loop between comparator 43 and the gain controlinput of attenuator 40. The selectors SW₁, SW₂, SW₄ are synchronized. Inposition I a signal representing the amplitude of the signal received bycoupler C is stored in store 44. In position II the signal representingthe amplitude of the output signal of the mixer X₂ is compared with thesignal in store and the difference is amplified and restores theamplitude to equality by means of the attenuator. To prevent anycoupling between the amplitude loop and the frequency loop, theamplitude loop pass band is wider than that of the frequency loop.

FIG. 12 shows an example of a frequency-to-voltage converter D₁ using afrequency discriminator 50 operating not on microwave but at a lowerfrequency, e.g. 150 MHZ. The discriminator 50 can be, for instance, ofthe FOSTER-SEELEY kind. It is preceded by a limiting amplifier 51, andso the amplitude loop can be omitted. Before the limiter a frequencychange is provided by a mixer 52 which receives the microwave inputsignals from selector SW₁ and a reference signal produced by anauxiliary controllable-frequency microwave oscillator 53. Its controlinput 22 receives the sum of the presetting voltage and the voltage ofthe slow control loop, as shown in FIGS. 8 and 9.

Linearization of the frequency controls of the oscillators in the caseof a single-frequency change receiver is not as critical as in the caseof double-frequency change receivers. The operating conditions of thesystem do not require the central frequency of the frequency-to-voltageconverter to be very stable (e.g. ± 2 to 3 MHZ) nor need thefrequency/voltage characteristic be particularly linear. Accuracydepends only upon the identity of the operating points on thefrequency/voltage characteristic during the two phases of storage andrecombination.

Of course, the invention is not limited to the embodiment described andshown which was given solely by way of example.

What is claimed is:
 1. A system for bringing the frequency of anoscillator under the control of the frequency of the carrier of a pulse,said system comprising means for measuring and storing a parametercharacteristic of the carrier frequency value throughout the duration ofthe pulse, means for measuring the same parameter in respect of thefrequency of the oscillator to be controlled, comparing means forcomparing, after the pulse, the second measured parameter with the firstand stored measured parameter, and control means for controlling thefrequency of said oscillator in function of the result of saidcomparison.
 2. A system according to claim 1, wherein said comparingmeans comprises a comparator having two inputs and an output coupled tosaid control means, and wherein said means for measuring and storing aparameter characteristic of the carrier frequency value of the pulse andfor measuring the same parameter in respect of the frequency of theoscillator comprises a frequency to voltage converter, a store connectedto one of the two inputs of the comparator, and two synchronizedselectors for applying to the converter input either the pulse or theoscillator output signal in the absence of pulse, and for connecting theconverter output either to the store or to the other input of thecomparator respectively.
 3. A system according to claim 1, wherein saidcomparing means comprises a comparator having two inputs and an outputcoupled to said control means, and wherein said means for measuring andstoring a parameter characteristic of the carrier frequency value of thepulse and for measuring the same parameter in respect of the frequencyof the oscillator comprises a frequency-to-voltage converter having twosymmetrical channels, the inputs of which are connected together, afirst and a second subtractor having their outputs respectivelyconnected to the inputs of the comparator, two stores connectedrespectively to the two inputs of the first subtractor and threesynchronized selectors, one of them being connected to the common inputof said channels for applying to said common input either the pulse orthe oscillator output signal after the pulse and the two other selectorsbeing connected respectively to the outputs of said channels forconnecting said two outputs either to the two stores respectively duringthe pulse, or to the two inputs respectively of the second subtractor,after the pulse.
 4. A system according to claim 3 of use if theoscillator frequency F1 differs from the pulse frequency F3 by aconstant value Fi further comprising a first frequency changing circuitconnected between the oscillator output and the respective input of thefirst selector and a second frequency changing circuit connected betweenan input terminal for the pulse and the respective other input of thefirst selector, the frequencies of the transposing signals generated insaid changing circuits and mixed with the signals of frequency F1 and F3respectively, differing from one another by the constant value Fi.
 5. Asystem according to claim 4, wherein the first frequency-changingcircuits has a controllable-frequency oscillator whose control input iscoupled with a circuit providing a slow control for bringing thefrequency of the oscillator to the mean value of the pulse frequency. 6.A system according to claim 5, wherein said slow control circuitcomprises in series a frequency-to-voltage converter, a sampling device,a storage circuit, and an integrator, the latter being connected betweenthe output of the frequency-changing circuit and the control input ofthe oscillator.
 7. A system according to claim 5, wherein said controlcircuit comprises an integrator connected between the storage circuitand the control input of the oscillator.
 8. A system according to claim2 wherein said frequency-to-voltage converter has a controllable centralfrequency and comprises a frequency control input, said system furthercomprising means for slowly controlling the central frequency of theconverter to the mean value of the pulse frequency.
 9. A systemaccording to claim 8, wherein the frequency-to-voltage converter is amicrowave frequency discriminator; said system comprising further anamplitude control loop which comprises a variable attenuator having again control input in series between the oscillator to be controlled andthe converter, an amplitude detector connected to the converter input,means for storing the detector output signal representing pulseamplitude, means for comparing the detector output signal representingthe oscillator signal amplitude with the stored signal, and a correctingamplifier in series between the comparator means and the attenuator gaincontrol input.
 10. A system according to claim 8, wherein thefrequency-to-voltage converter comprises a frequency discriminatorpreceded by a limiting amplifier and by frequency-changing means in theform of a mixer and of an auxiliary controllable-frequency oscillatorreceiving the presetting signal and the slow control signal.
 11. A pulseradar receiver using the system according to claim 7 comprising inseries two frequency-changing circuits for the received signal, andwherein the first such circuit comprises a mixer connected to theslow-control-circuit oscillator, and the second frequency-changingcircuit comprises a mixer connected to the automatic frequency controlcircuit oscillator.
 12. A pulse radar receiver using the systemaccording to claim 8, comprising a frequency-changing circuit for thereceived signal, and wherein such circuit comprises a mixer connected tothe controlled oscillator.
 13. A pulse radar receiver using the systemaccording to claim 9, comprising a frequency-changing circuit for thereceived signal, and wherein such circuit comprises a mixer connected tothe controlled oscillator.
 14. A pulse radar receiver using the systemaccording to claim 10 comprising a frequency-changing circuit for thereceived signal, and wherein such circuit comprises a mixer connected tothe controlled oscillator.
 15. A pulse radar receiver according to claim12 comprising a magnetron delivering in additional to the transmissionpulses a copy signal representing the transmitted frequency, and whereinthe automatic frequency control system comprises a first adder foradding the copy signal to the frequency control signal of the controlledoscillator produced by the control circuit and a second adder for addingthe copy signal to the frequency control signal of thefrequency-to-voltage converter produced by the slow control circuit.